EDIT:
Ok i might have missed something - in an outgoing transaction .. do i specify which incoming transaction is used to "spend" my btc?
So in this case A and B give a btc to C - C now has two btc which he can spend. If C now gives D a bitcoin - does C specify in the transaction WHICH transaction(s) he uses as input to give D the btc? And is this probably transparent in the blockchain? That would ofcourse change how i have to think about the whole picture.
Yes, in an outgoing transaction you specify the inputs. This is usually done automatically by your wallet software. However, if for example, you enable the Coin Control feature of Bitcoin Core, you would be able to explicitly specify which inputs are used for your outgoing transaction.
I use coin control all the time but its very annoying and not user friendly. I hope in the future they can figure out a way to automatically mix the coins so you don't have to worry about coin control and taking into account inputs and outputs otherwise we will never reach mainstream usage.